Various systems have been developed for measuring head movement ("head tracking") and the gaze of an eye ("gaze point tracking") with respect to a video display terminal or other display device. In many such systems, a video signal corresponding to an image of a reference point is received as a video signal that is digitized and stored in a memory called a frame buffer. Because the video signal corresponding to a single image contains approximately 500 rows by 500 columns of individual pixel signals, a large memory is required for the frame buffer. The stored image is then processed to identify and locate objects within the image.
Image processing in this way has many drawbacks. First, an entire array of pixels for an image must be digitized and stored, requiring a large memory for the frame buffer. Processing the digitized image data is complex and it is difficult to identify images corresponding to the head or eye in the image. In addition, if image processing is to be fast enough to permit head or eye tracking, expensive signal processing systems are necessary. These problems are particularly severe in cost-sensitive applications. Accordingly, improved methods and apparatus are needed for simple, inexpensive image processing.